Alma stresses again his point that a law, to be a genuine law, must have consequences, or punishment for breaching it. As I noted above, “Don’t touch this book” is not a law, even though it is prohibitive language. Law is created by affixing punishment.
Alma’s argument that punishment is essential to Yahweh’s scheme interestingly implies that there is no sin without law and no law without punishment. In this condition, justice has no realm of action, because there is nothing to judge against. Justice cannot hold us liable for the violation of something that doesn’t exist.
Similarly, if no soul is in danger of “punishment” because of law, “mercy” cannot act, because it cannot save us from something that isn’t happening. If we are not subject to “punishment,” there is no way for mercy to save us from it. Once again, Alma is demonstrating the absolute requirement of both justice and mercy, with law/punishment forming one part of the plan and mercy/repentance/ atonement forming the other. Without stating it, he is also underscoring the problem of Adam and Eve in the Garden of Eden. In that state they were without thie opposition that creates the conditions in which justice and mercy may act.